This invention relates generally to a method for roll embossing metal strip. More particularly, the invention relates to a method that results in uniform and predictable edge thicknesses and prevents the formation of tensile stresses in the roll embossed strip, thus assuring good weldability at the edges of the strip.
One method of manufacturing metal tubing, e.g. copper or aluminum tubing used in tubed liquid-to-air or liquid-to-liquid heat exchangers, is by roll forming and welding metal strip. The strip is first longitudinally roll formed into a circular cross section in a tube forming mill. The opposite edges of the strip, constituting after roll forming a longitudinal seam, are then welded to form the tube. The two strip edges must be of a uniform and adequate thickness in order to obtain a good seam weld.
Grooving or ribbing the interior or exterior surfaces of a heat exchanger tube usually improves the heat transfer efficiency of the tube so enhanced. The specific mechanisms by which the heat transfer performance of a given tube is improved by surface enhancements vary with specific applications. In general, however, tube surface enhancements increase heat transfer performance by increasing the heat transfer area of the tube and by promoting fluid flow conditions that are conducive to heat transfer between the fluids in contact with the surfaces of the tube.
There are numerous techniques for enhancing the surface or surfaces of a tube. One method, particularly suitable for enhancing tubing formed from rolled and welded strip, is to place the enhancements on one or both sides of the strip before the strip is formed into a tube. In a modern high speed tube manufacturing process using continuous strip feedstock, the surface enhancing can be accomplished by placing a roll embossing machine downstream of the strip supply and upstream of the tube forming mill. The roll embossing machine can then form surface enhancements on a sides or sides of the strip before the strip enters the tube forming step of the process.
Roll embossing of a strip can result in an elongation of the strip. For example, one length unit of metal feedstock entering the inlet of the roll embossing machine can exit the outlet of the machine as 1.2 length units of embossed strip. It may be that the minimum thickness of the strip in the area that is embossed will provide adequate wall thickness in the finished tube but is not adequate to provide for good weld strength in the weld zone at the strip edge. In this case, a weld zone in which there is no surface enhancement is left at each edge of the strip. If these weld zones receive no treatment in the roll embossing machine, they will not be elongated.
If the central portion of the strip is embossed, and therefore elongated, but the weld zones at the edges are not, tensile stresses will be introduced into the weld zones. These stresses may become relieved by deformation of the strip edges in unpredictable ways, resulting in nonlinear edges with nonuniform thicknesses. Such discontinuities and nonuniformities will result in difficulties in presenting a consistent, predictable seam to the seam welder and thus can adversely impact the ability to get a good weld joint during later seam welding of the roll formed strip into finished tubing. This problem is particularly serious in the high speed, automatic welding processes used in modern tubing manufacturing lines.